I am seriously considering purchasing a sigma 50-500 mm telephoto lense for my olympus E500 camera , much,much cheaper than the olympus version.Does anyone have experience of using such a lense for birding, bearing in mind the fairly heavy weight and the fact of not being able to use a tripod for most shots.Would appreciate some opinions as the lense is still quite pricey at £750 and I dont want to buy unsuitable gear.